Photovoltaic cells have long been considered a possible alternative to nonrenewable energy sources. Most types of photovoltaic cells have a photoactive area (i.e., an area that generates electricity in response to light) that is composed of a semiconductor layer disposed between two electrode layers. The various layers are typically deposited on a flat, rigid supporting substrate, such as glass. A number of electrically interconnected photovoltaic cells may be supported on a common supporting substrate. The combination of the supporting substrate and the photovoltaic cell(s) is typically referred to as a photovoltaic module.
One process for making a photovoltaic module is to sequentially form layers on top of a supporting substrate to build a photoactive area on the supporting substrate. The photoactive area may then be divided into a number of individual photovoltaic cells which may be interconnected in series to boost the voltage output from the photovoltaic module. The photovoltaic cells may then be encapsulated to protect the cells from the external environment during operation of the module. The process of building a photovoltaic module by the sequential formation of layers on a supporting substrate is a common technique for making thin film photovoltaic devices, such as those using thin semiconductor films of cadmium telluride or copper indium diselenide in a power-producing semiconductor layer.
The sequential deposition of layers on a supporting substrate such as glass, however, is expensive. Some deposition steps may require a high temperature, which requires energy to heat the supporting substrate and induces thermal stresses into the supporting substrate which may cause breakage of some substrate materials, such as glass. Also, the processing of glass sheets is cumbersome and not well suited to a high speed operation.
Furthermore, for some thin film semiconductor materials, such as cadmium telluride, the semiconductor material is typically recrystallized following deposition of the semiconductor material to obtain crystals of a suitable size and morphology for use in a photovoltaic cell. Recrystallization tends to be, however, a very slow process and requires a significant amount of energy to heat not only the desired semiconductor film but also the supporting substrate, significantly adding to the expense of the manufacturing process. Additionally, during recrystallization, void spaces tend to form in the semiconductor film, making the semiconductor film more susceptible to electrical shorting through the film.
Another problem encountered with many thin film photovoltaic devices is control over the stoichiometry of the components of the semiconductor material during deposition of semiconductor thin films. For example, maintenance of the stoichiometry of copper indium diselenide during deposition of a thin film can be difficult to control, especially in a high volume industrial setting.
There is a need for an improved process for making photovoltaic devices, and especially for making thin film devices, which is more suitable for high volume production and which reduces the costs associated with processing the supporting substrate.